The present invention generally relates to gas cylinder springs and, more particularly, to gas cylinders which are supplied gas from a manifold into which the gas cylinders are threaded.
Gas cylinders of the type disclosed in this application typically employ a compressible gas, such as nitrogen gas, to resist the movement of a piston within a cylinder and thereby cushion the plenums of a press. These cylinders are generally of two basic types: threaded body and stand-alone cylinders. Stand-alone cylinders are self-contained gas springs which are charged with a quantity of compressible gas, such as nitrogen gas and then sealed. Stand-alone springs are placed in various openings in the plenums of a press to cushion the movement of the press.
Threaded body cylinders have a cylinder body that is open at one end with a piston rod extending out of the other end. The opened end has external threads that permit the spring cylinder to be threaded into a manifold which contains a constant supply of pressurized nitrogen gas. The manifold generally has a number of threaded openings for receipt of a number of cylinder springs. A supply of nitrogen gas is supplied to the manifold and to the spring cylinders at a predetermined pressure.
Both types of gas springs work in the same manner. A piston reciprocates within a cylinder body against a compressible gas, such as, for example, nitrogen gas. The cylinder is pre-charged with about 2,000 psi of nitrogen gas. When the piston is forced into the cylinder body, the nitrogen gas is compressed to a maximum pressure of about 3,000 to 5,000 psi, depending upon the cylinder and piston size.
Threaded body cylinders have specific difficulties which are not encountered in standalone cylinders. One problem with threaded body cylinders is when the pressure within the cylinder steadily creeps upwardly during operation. This pressure creep is caused by the leaking of atmospheric air into the gas cylinder on the downstroke of the piston. The atmospheric air leaks past the seals surrounding the piston rod. During the downstroke of the piston, a vacuum is formed, which pulls atmospheric air into the gas spring. Due to the nitrogen gas being supplied at a predetermined pressure, this pressure leaks around the seals and causes the pressure within the cylinder to constantly creep upwardly. This increased pressure has an adverse affect on the seals in that greater pressures begin to act upon the seals, causing greater wear, and in many cases, a breakdown of the seals. As the atmospheric air continually leaks around the piston head seal, the cushioning effect of the gas cylinder is substantially changed, and, in many cases, the pressure can become dangerously high.
A further difficulty in threaded body cylinders is the ingress of contaminants into the cylinder body. As atmospheric air is drawn into the cylinders, contaminants flow with the atmospheric air and begin to accumulate along the walls of the cylinder and ultimately scar the cylinder walls as the piston head reciprocates with respect to the cylinder walls.
One method of controlling this increased pressure is to vent it through the piston head and into the manifold. U.S. Pat. No. 4,342,448 discloses a threaded gas cylinder that employs a valve in the piston head that allows the pressure above the piston head to be released below the piston head when the pressure reaches a predetermined value. There are numerous problems with this type of venting system. As would be expected, contaminants flow with the pressure that flows through the valve. The contaminants are eventually forced into the manifold. It has been found by Applicant that many manifolds are heavily contaminated with environmental contaminants found in the environment wherein such threaded body cylinders operate. It is believed by applicant that the manifolds are the final resting place for the contaminants due to the ever-increasing pressure within the threaded body cylinder forcing the contaminants through the cylinder into the manifold.
The contaminate build-up in the manifold is particularly troubling when one or more of the gas cylinders is changed and the system recharged. To change the cylinders, the system pressure is released, one or more cylinders changed, and then the system is pressurized. When the system is pressurized, contaminants are believed to flow into the cylinders. This leads to scarring of the cylinder walls and greatly reduces the life of the cylinders.
Another problem with this type of venting system is the potential for contaminants to remain in the lower chamber of the cylinder before they are deposited into the manifold. Again, the presence of contaminants in the cylinder creates scarring of the cylinder walls and reduces the life of the cylinders.
A still further problem with the valve system disclosed in the '448 patent is the tendency for the contaminants to interfere with the operation of the spring which affects the ability of the cylinder to relieve pressure, as needed.